Michael Jackson has been taken by ambulance to a Los Angeles hospital after reportedly going into cardiac arrest, TMZ reports. The 911 call from Jackson's Holmby Hills home was placed at 12:21 p.m. PST. No other details are known as yet, but Jackson's mother is reportedly already on the way to the hospital. Jackson is in "really bad shape," an unspecified family member told TMZ, while Jackson's father Joe told the site, "He is not doing well." Los Angeles Fire Department Captain Steve Ruda told L.A. TV station KTLA that Jackson wasn't breathing when paramedics arrived; they performed CPR and took Jackson to UCLA Medical Center.

Jackson is in the Los Angeles area for rehearsals for his upcoming 50-show This Is It! run of concerts at London's O2 Arena. As Rolling Stone reported, sources say Jackson has been absent from many rehearsals, with a stand-in required to take his place. While no reasons were given for why Jackson might be missing the rehearsals, questions regarding the King of Pop's health have dogged the Thriller singer for years.

So far, the four opening This Is It! shows have been postponed due to the "massive and technically complex" nature of the shows and not because of Jackson's health, a rep for AEG Live, who is staging the O2 concerts, told Rolling Stone. No official confirmation or statement regarding TMZ's report was available at press time.